1. Field of the Invention
The present invention relates to a particle beam irradiation apparatus for use in medical treatment to accelerate and irradiate a particle beam. The present invention also relates to a safety device for the particle beam irradiation apparatus.
2. Description of the Related Art
In a particle therapy system, an accelerator accelerates a charged particle beam, i.e., a particle beam (such as a proton beam or a heavy charged-particle beam (e.g., a carbon ion beam)), and the accelerated charged particle beam is used for treatment of cancers (diseased parts). The particle therapy system includes an irradiation nozzle for irradiating the charged particle beam to a cancer. The particle beam will be referred to as an “ion beam” hereinafter. The irradiation nozzle spreads the ion beam in match with the cancer in a direction perpendicular to the direction of advance of the ion beam. This process is called spread of an irradiation field. For the spread of the irradiation field, there are known a scattering method (see Non-Patent Reference 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8 (August 1993), pp. 2076-2086), and a wobbling method (see Patent Reference 1; JP,A 2000-202047, and Non-Patent Reference 1). According to the scattering method, a scatterer is installed in a beam path within the irradiation nozzle to enlarge the ion beam through scattering by the scatterer. According to the wobbling method, a pair of scanning magnets provided in the irradiation nozzle are energized to scan the ion beam so as to draw a circle, thereby spreading the ion beam. In some cases, both the scattering method and the wobbling method are used in a combined manner. The irradiation nozzle further includes a SOPB (spread-out of Bragg peak) device (e.g., a ridge filter or a range modulation wheel) for spreading out an energy distribution to increase the range in the direction of advance of the ion beam, a fine degrader for adjusting the energy of the ion beam to final irradiation energy, a collimator for cutting the ion beam that is not necessary for the irradiation, a dose monitor, and a beam position monitor.
The energy (namely, range) of the ion beam irradiated to the cancer can be measured by a water phantom device (see Patent Reference 2; JP,A 11-64530) attached in the irradiation nozzle. As another method, the energy of the ion beam can also be measured by laminating metallic plates and detecting, from each of the metallic plates, the amount of charges lost through the metallic plates when the ion beam passes them.